Geochemistry of fluid inclusions and hydrothermal alteration in vein- and fracture-controlled mineralization, stockwork molybdenum deposits

Molybdenum mineralization and coextensive alteration at Questa, Hudson Bay Mountain, and Endako occur in single/composite veinlets which exhibit distinctive alteration assemblages and paragenesis. Fluid inclusion populations from each veinlet type are compositionally distinct. Early fluorine rich, biotite-stable alteration is associated with hypersaline brines. These inclusions homogenize most frequently by halite dissolution at temperatures from 350°-600°+C. Molybdenum mineralization also coincides with quartz-sericite-pyrite alteration and inclusions having moderate to high salinity and lower (≤ 350°C) temperatures of entrapment. Ubiquitous low-salinity inclusions in addition to inclusion types which characterize each veinlet type suggest superposition of meteoric-dominated convective circulation. These data are evidence for magmatic fluids, separate in both space and time from ingress of meteoric hydrothermal solutions. Hypersaline brines were precursors to those solutions which precipitated distinctive alteration associations, and formed by boiling/condensation of fluids released by episodic fracturing and ensuing adiabatic decompression. Highly variable and complex compositional zoning characterizes (K,Na)-feldspar, biotite, and muscovite solid solutions Although zoning within individual grains is common, composition al trends in the averaged compositions can be correlated with both position in the emplacement sequence and textural characteristics of these alteration phases. Alkali feldspars coexisting in veinlets constitute sensitive geothermometers which pro- vide reliable depositional temperatures (≥ 350°C). X(annite) is highest in magmatic biotite, intermediate in biotite which exhibits replacement textures, and significantly lower in veinlet assemblages. X(fluorphlogopite) systematically increases from magmatic to veinlet biotite associations; f(H₂O/HF) ratios computed from these compositions predict log f(HF) of approximately -4.0. Departures from muscovite stoichiometry persist among different veinlet assemblages and textural associations, but were not sampled with enough regularity to define trends in spatial disposition. Alteration geochemistry indicates that mineralization occurred from solutions equilibrated with veinlet assemblages. Fluid-mineral equilibria computations imply chemical characteristics of the ore-forming solutions similar to those measured in high-temperature geothermal fluids. Molybdenite solubility shows extreme dependence on temperature, pH, f(O₂), and f(HF). Molybdenum concentrations up to 100 ppm are predicted in oxidized solutions at 350°C. In near-neutral solutions significant amounts of molybdenum are transported as MoO₃F⁻ and HMoO₄⁻, with lesser amounts as H₂MoO₄⁰ and MoO₂⁺. Chloride and sulfide complex concentrations are not significant. MoO₃F⁻ becomes the primary transporter of molybdenum in acid solutions and high f(HF).Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat